The present invention relates to the field of electronic image reproduction, and to a method for calibrating image recording equipment in which raster areas, such as images or chromatic components are recorded on a recording medium, for example on film or on a printing plate, pixel-by-pixel, in lines, by use of at least one exposure beam which is produced in an exposure unit.
German Patent DE 43 03 081C describes a method and a device for exposure calibration of recording equipment, in which so-called contone data are recorded by an exposure unit. The term contone data refers to image data with a resolution of, for example, 8 bits and which has been obtained by sampling or scanning half-tone images. The calibration can be carried out in the exposure unit itself or in an upstream raster image processor (RIP) in which the image data are prepared for recording. The purpose of the calibration is to correct the exposure characteristic of the exposure source and the film characteristic, such that the raster points are reproduced in the correct area coverage on exposure. The calibration is no problem with contone image data. If, for example, 50% of the image is blackened, and the aim is also to expose the image with 50% area coverage, a look-up table, which contains the correction values, is produced for linearization. A step wedge is exposed and measured using the image recording equipment in order to produce the look-up table. This confirms what correction values are required in order to produce a specific area coverage on the film. The look-up table is used to vary the drive to the exposure source such that the film is blackened with 50% area coverage. This thus results in linearization that allows the image recording equipment to record the correct tone levels of the image. However, this method does not allow calibration of the recording equipment to be carried out when the recording equipment is actuated using line data, for example using binary data (bit map). The resolution of this bit map corresponds to the resolution of the exposure beam. Line data are image data which have been obtained by sampling or scanning line drawings, text, graphics or raster images, and are stored in the form of a bit map, with the raster points likewise being in the form of line data. However, the linearization process for contone data cannot be applied to such line data, since the contone data would have 8-bit resolution.
Published, European Patent Application EP 0 734 151A1 describes a method for producing a frequency-modulated raster, in which the smallest raster points vary somewhat in size and shape. This minimizes the losses of the printing range in the very bright and very dark tones, which are caused by overexposure during film exposure and by dot growth during printing, and adequately fine graduations of the gray levels are maintained in these tonal-value areas. However, the method is not suitable for tonal-value calibration of a recording appliance that is actuated using line data.
It is accordingly an object of the invention to provide a method for calibrating image recording equipment that overcomes the above-mentioned disadvantages of the prior art methods of this general type.
The invention is based on the object of specifying a method for calibrating image recording equipment, by which the recording equipment can be calibrated even when the image recording equipment is actuated using line data. A further object of the invention is to be able to carry out gradation corrections in order to correct the image.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for calibrating image recording equipment on a basis of a previously determined raster percentage value correction function, in which raster areas are recorded in a form of raster points on a recording medium, pixel-by-pixel, in lines by at least one exposure beam produced in an exposure unit. The method includes the steps of splitting an image, which is stored as a bit map and is composed of line data, into fields; averaging each field for determining a raster percentage value; determining a change to the raster percentage value from the raster percentage value correction function; and carrying out the change to the raster percentage value, depending on a change direction, by one of erosion and dilatation of raster points in the field.
In accordance with an added feature of the invention, there are the steps of calculating parameters required for the change in advance and storing the parameters in a change table; and reading the parameters associated with the raster percentage value from the change table.
In accordance with an additional feature of the invention, there is the step of splitting the image into fields of equal size.
In accordance with another feature of the invention, each of the fields covers approximately one raster grid interval.
In accordance with a further feature of the invention, there is the step of using a surrounding area which is larger than a field size for the averaging.
In accordance with another added feature of the invention, there is the step of carrying out operations of the erosion and the dilatation more than once.
In accordance with another additional feature of the invention, there is the step of interchanging the erosion and the dilatation operations after inverting the line data of the image.
In accordance with a further added feature of the invention, there is the step of applying the erosion and the dilatation to duplicated line data in order to increase accuracy.
In accordance with a further additional feature of the invention, there is the step of calculating the change table for the raster percentage value in fixed steps.
In accordance with yet another feature of the invention, an entry in the change table contains at least one parameter selected from raster percentage values, desired changes, duplicating factors, number of erosions, number of dilatations, a type of operation and a result.
In accordance with a further feature of the invention, there is the step of entering a combination of parameters in the change table which gives a best approximation to a desired change.
In accordance with another feature of the invention, there is the step of carrying out an edge treatment when the erosion and the dilatation are carried out.
In accordance with a concomitant feature of the invention, there is the step of returning the line data to an original resolution after carrying out the erosion and the dilatation on duplicated data.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for calibrating image recording equipment, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.